WEBVTT NOTE Transmission Lines as Circuit Elements 1 00:00:00.000 --> 00:00:01.620 Velocity factor. 2 00:00:01.620 --> 00:00:06.543 Question. What is the velocity factor of a transmission line? 3 00:00:07.443 --> 00:00:16.043 Answer. The velocity of a wave in the transmission line divided by the velocity of light in a vacuum. 4 00:00:16.643 --> 00:00:23.987 Question. Which of the following has the biggest effect on the velocity factor of a transmission line? 5 00:00:24.887 --> 00:00:29.415 Answer. The insulating dielectric material. 6 00:00:30.015 --> 00:00:46.295 The dielectric, not the conductor. Solid polyethylene gives about 0.66; foam about 0.80; air-spaced parallel line about 0.95. 7 00:00:46.295 --> 00:00:53.745 Question. Why is the electrical length of a coaxial cable longer than its physical length? 8 00:00:54.645 --> 00:01:01.925 Answer. Electromagnetic waves move more slowly in a coaxial cable than in air. 9 00:01:02.525 --> 00:01:14.305 Slower propagation means more cycles fit in a given physical distance, so the cable is electrically longer than it looks. 10 00:01:14.305 --> 00:01:32.219 Worked, as the pool asks it: the physical length of an air-insulated parallel conductor line that is a half wavelength at 14.10 MHz. Free-space half wavelength = 150 / 14.10 = 10.64 m. 11 00:01:32.219 --> 00:01:51.885 With a velocity factor of about 0.95, nearly one, since it is air-insulated, the answer is close to 10.6 metres. That the answer is so nearly the free-space figure is the point of an air-insulated line. 12 00:01:51.885 --> 00:02:05.885 Stubs: the four lengths. A length of line with its far end shorted or open behaves, at the near end, as a reactance, and the behaviour cycles every half wavelength. 13 00:02:05.885 --> 00:02:24.324 Four cases, all asked: Length, 1/8 wavelength; Shorted far end, inductive reactance; Open far end, capacitive reactance. Length, 1/4 wavelength; Shorted far end, very high impedance; Open far end, very low impedance. 14 00:02:24.324 --> 00:02:39.092 Length, 3/8 wavelength; Shorted far end, capacitive; Open far end, inductive. Length, 1/2 wavelength; Shorted far end, very low impedance; Open far end, very high impedance. 15 00:02:39.092 --> 00:02:55.910 Rather than memorise eight cells, hold two rules: A quarter-wave line inverts. Short becomes open, open becomes short. That is also why the quarter-wave transformer works. A half-wave line repeats. 16 00:02:55.910 --> 00:03:06.922 Whatever is at the far end appears at the near end. A shorted half wave looks like a short; an open half wave looks like an open. 17 00:03:06.922 --> 00:03:21.263 The eighth-wave cases are halfway to the inversion, so they are pure reactances rather than extremes, and their sign follows from which extreme they are heading toward. 18 00:03:21.263 --> 00:03:32.446 The practical payoff: a shorted quarter-wave stub is a very high impedance at its design frequency and a short circuit at twice it. 19 00:03:32.446 --> 00:03:47.983 Hang one on a transmitter's output and it passes the fundamental while shorting the second harmonic to ground. That is a harmonic filter made of a piece of coax. Line types compared. 20 00:03:47.983 --> 00:03:56.623 Question. How does parallel conductor transmission line compare to coaxial cable with a plastic dielectric? 21 00:03:57.523 --> 00:04:00.523 Answer. Lower loss. 22 00:04:01.123 --> 00:04:09.393 Parallel line has more conductor surface, more air dielectric, and no lossy jacket in the field. 23 00:04:09.393 --> 00:04:24.814 It is genuinely better, and it must be kept clear of metal and of people, cannot be buried or run through a wall, and radiates if unbalanced. The trade is real in both directions. 24 00:04:24.814 --> 00:04:41.882 Question. Which of the following is a significant difference between foam dielectric coaxial cable and solid dielectric coaxial cable, assuming all other parameters are the same? A. 25 00:04:41.882 --> 00:05:01.025 Foam dielectric coaxial cable has lower safe maximum operating voltage. B. Foam dielectric coaxial cable has lower loss per unit of length. C. Foam dielectric coaxial cable has higher velocity factor. D. 26 00:05:01.025 --> 00:05:03.854 All these choices are correct. 27 00:05:04.754 --> 00:05:08.274 Answer. All these choices are correct. 28 00:05:08.874 --> 00:05:10.941 The last one is the catch. 29 00:05:10.941 --> 00:05:26.519 Foam is better electrically and worse at keeping water out once the jacket is breached, which, given that moisture contamination is the leading cause of coax failure, is not a small consideration. 30 00:05:26.519 --> 00:05:30.016 Question. What is microstrip? 31 00:05:30.916 --> 00:05:41.904 Answer. Precision printed circuit conductors above a ground plane that provide constant impedance interconnects at microwave frequencies. 32 00:05:42.504 --> 00:05:51.378 At microwave, a track on a circuit board is a transmission line whether you designed it as one or not. 33 00:05:51.378 --> 00:06:06.602 Microstrip is the discipline of designing it deliberately, controlled width, controlled substrate, controlled impedance. Check yourself. A shorted quarter-wave stub at 14 MHz. 34 00:06:06.602 --> 00:06:24.784 What does it look like at 14 MHz, and at 28 MHz? You need a half-wavelength phasing line at 21 MHz using coax with a 0.66 velocity factor. Physical length? Why is foam coax not automatically the better choice? 35 00:06:30.784 --> 00:06:51.203 Very high impedance at 14 MHz (a quarter wave inverts the short), and a short circuit at 28 MHz (where it is a half wave, which repeats the short). Free-space half wave = 150 / 21 = 7.14 m; times 0.66 = 4.71 metres. 36 00:06:51.203 --> 00:07:07.064 It absorbs more moisture once the jacket is breached, and moisture contamination is the leading cause of coax failure, against lower loss and a higher velocity factor.